One standard limiter circuit of the prior art is a simple circuit of two parallel diodes which clamp a signal when the signal becomes high enough to turn either one of the diodes on. An example of this structure is found in Operational Amplifiers and Linear Integrated Circuits, page 285 (Prentice-Hall 1977).
Another limiter circuit uses a bridge structure for which the clamp level threshold is set by biases on diodes.
These and other known limiter/clamp circuits typically do not solve certain problems that arise occasionally in commercial telecommunications, high definition television and in other arts which utilize FM transmission technology and, therefore, require limiter/clamp functions.
For example, it often is necessary to provide a clamp circuit that clamps at a lower voltage, such as 0.4 volts; whereas typical standard clamp circuits operate, for example, between 0.7 and 0.8 volts and the clamp characteristic follows that of a diode.
It also is useful to maintain symmetric clamp characteristics. This is needed in applications where timing information is extracted from zero crossings (where a voltage changes from negative to positive); and is desirable in other situations because the related circuitry would be less complex. For instance, in a situation where the harmonics generated from the normal limiting action of this type of circuit must be filtered out, the required filter would be less complex because the first overtone to eliminate would be the third harmonic, rather than the second which results from asymmetric performance.
Other performance characteristics occasionally necessary in limiter/clamp circuits, but which are not present in a working combination in available limiter/clamp circuits, include obtaining an absolute clamp level and maintaining linear performance for normal input levels. Prior art circuitry does not provide separate control of the linear range and clamp level. Typically, the clamp level is trimmed-in and the resulting linear range performance is simply accepted. Prior art usually produces harmonic distortion products even for low-level signals.